In order to satisfy the last-mentioned condition, the Applicant has already proposed devices in which translation motion of the two portions of a coupling towards each other serves to assemble a tube with an endpiece, sealing being obtained by means of a gasket that is associated with one of said portions, see FR-A-2 632 707 and FR-A-2 656 404, for example. Although such devices give satisfaction, they are nevertheless sometimes considered as being too expensive for fitting to the engines of all the vehicles with which they could be associated, and this is because of the presence of a gasket of a structure that is relatively complex and to the presence of means distinct from the gasket for coupling together the two portions of the coupling. In FR-2 636 713, proposals have also been made for a junction device between a flexible hose and a rigid endpiece in which a coupling piece made of plastic and secured to the flexible tube has fastening means for engaging the rigid endpiece, and two sealing lips for cooperating with said endpiece, which endpiece must then necessarily both be relatively complex in shape and be made with dimensional characteristics that are very accurate, thereby increasing cost.